Don't forget to tell our users that the epoxy GL functions are __dllimport.

This lets the compiler generate faster function calls (call through
function pointer, instead of call into a linker-generated stub func
containing jump to function pointer).
win32: Use thread local storage to get us a dispatch table per thread.
win32: Start wrapping the 1.0 wgl functions.

I want to have a chance to intercept wglMakeCurrent for WGL's
context-dependent function pointer handling.
win32: Move dynamic symbol loading to do_dlsym().

This should help get us better error handling when we accidentally
call the wrong loader path.
Drop the PLATFORM_HAS_EGL check in epoxy_egl_dlsym().

If you accidentally call it on a non-egl platform, better to get an
informative error message.
win32: Reset all dispatch tables to re-resolve on any wglMakeCurrent.
win32: Expose the dispatch table reset funtion in the public API.

Fixes #2.
win32: Convert the API to being based on function pointers, like Linux.

For performance, I want to be able to make single-context (well,
single-pixel-format-and-device) apps be able to directly call GL
functions through function pointers.  Bake that into the ABI now so I
can get a release out the door and fix this up later.

This also fixes the lack of __stdcall annotation on the
PFNWHATEVERPROC typedefs.